Sheet basis weight detection in apparatus which perform operations on sheets, such as printers and image reproduction equipment, is desirable because it allows the range of paper weights processed by the apparatus to be increased. The quality of the output of basis weight-sensitive work stations can be maintained by adjusting processing parameters in accordance with the basis weight of the sheet being processed. Such basis weight determination, which bears a linear correlation to sheet thickness, and consequent adjustment can also increase the life of machine components and reduce machine down time resulting from jamming of improperly fed sheets.
Optical sensors for detecting sheets fed along a feed path to be processed in a plurality of work stations are known. Such sensors are shown, for example, in U.S. Pat. No. 4,670,647 to Hubble et al, owned by the assignee of the present application. In such systems, an analog sensor comprising a light emitting diode (LED) and a phototransistor is used in ON/OFF digital fashion by coupling the output of the phototransistor to a threshold device, such as a Schmitt trigger. In this configuration, the sensor is utilized for detecting the presence of a sheet at a particular point in the feed path to provide timing and control signals for subsequent sheet handling. In addition, the sensors can be used as jam detectors to detect the presence of a non-moving sheet in the feed path. Under jam conditions, the output signal of the sensor is used to terminate downstream operations until the jam is cleared. However, this system does not detect the basis weight or the thickness of the paper.
A variation of the foregoing arrangement is described in U.S. Pat. No. 4,151,410 to MacMillan et al. This arrangement senses rapidly fluctuating output levels of the phototransistor brought about by variable translucency of the fed sheet to indicate proper movement of the sheet. When the rapid fluctuations stop or when there is a rapid drop in sensor output which results from double feeding of the sheet, a signal indicative of a paper jam is given by the electronic detection system. However, this arrangement does not include the capability of determining a thickness or basis weight of the sheet from the sensor signal.
Another method for determining thickness of thin sheets is shown in U.S. Pat. No. 4,937,460. However, systems of this type rely on contact of the sheet by a receiver or similar element to provide a variable analog signal indicative of paper thickness. However, such systems have the disadvantage of requiring an element to engage the sheet and such engagement introduces the chance for the detecting system itself to influence the detected sheet thickness. Further, such arrangements require additional hardware for carrying out the thickness measuring operation. This adds to the cost, complexity and size requirements for equipment employing such thickness detection apparatus.